Sayyiduna Abdullah bin Amr bin al-As (may Allah be pleased with them both) said that the Prophet (peace and blessings be upon him) said: "This religion is firm, so enter into it gently, and do not make the worship of your Lord hateful to yourself, and do not push ahead so far that you lose your goal. Act like a man who thinks he will never die, and be as cautious as one who fears that death may come to him tomorrow."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4744